How to Add existing Scene to VSE?
A ".blend" file is a an entire project that consists of 1 or more scenes.

A .blend file can have a scene dedicated to VSE output.

Any .blend project can "Append" another .blend's scenes.







Navigate to the required .blend project and select the Scene category.

Once you have appended or "imported" an external scene it is available to add to the timeline (shift-A) in the first .blend file's VSE scene as a scene strip.